Dual focus cathode-ray tubes



y 2, 1956 H. s. ALLWlNE 2,747,134

DUAL FOCUS CATHODE-RAY TUBES Filed April 30, 1953 2 Sheets-Sheet lHHRRISDNSHLLWNE ATTORNEY INVENTOR. Haamsnm S. H'LLWIN ATTORNEY z wlafk 2Sheets-Sheet 2 1956 H. s. ALLWlNE DUAL FOCUS CATHODE-RAY TUBES FlledAprll 30 1953 DUAL FOCUS CATHODE-RAY TUBES Harrison S. Allwine, Trenton,N. .L, assignor to Radio Corporation of America, a corporation ofDelaware Application April 30, 1953, Serial No. 352,239

11 Claims. (Cl. 315-13) This invention relates to improvements incolor-kinescopes and other cathode-ray (C. R.) tubes of the kind whereinelectrons are subjected to a first focusing action near their source orvirtual source and to one or more independent focusing actions near thescreen or target of the tube.

In cathode-ray tubes of the kind wherein the beamelectrons approach aviewing screen through the apertures of a nearby grill or mask theelectrons are subjccted to a focusing (and/ or shadowing) action at ornear the screen which is independent of the focusing action that takesplace at or near the gun of the tube. The influence to which theelectrons are subjected at" or near the screen differs in differenttypes of screen units. For example, in the shadow-mask dot-screen tubesof Goldsmith 2,630,542 (wherein the mask and screen are ordinarilyoperated at the same potential) the screen-unit subjects the electronsto a focusing action which may be said to be similar to that which takesplace in a pin-hole camera. On the other hand, if a dot screen and itsmask are operated at different potentials, with the screen at the higherpotential, the beam electrons are subjected to the converging action ofa spherical electron-lens system. Similarly, if a line screen isoperated at a higher potential than its parallel-Wire grill (as inFlechsig, French Patent 866,065 of 1941) the beam electrons aresubjected to the converging action of a cylindrical lens system.

If a line type screen-unit is provided with more than one grill, theelectrons may be subjected to the action of a compound cylindrical lenssystem. If the grill wires are properly oriented (for example, with thewires of one grill at right angles to the color lines on the screen, asdisclosed by Edward G. Ramberg in copending application Serial No.277,182, now U. S. P. 2,728,044) the beamelectrons may be subjected to aconverging action in one plane and an independent diverging action in aplane at right-angles to the first.

In color-kineseopes containing plural or multi-element lenticularscreen-units the particular screen-color that is illuminated at anygiven instant is normally a function of the particular angle at whichthe beam electrons approach the screen. In the case of a tri-colorkinescope, three-guns may be employed; i. e., one gun for eachscreen-color. In such 3-gun C. R. tubes the greater the angularseparation of the guns the easier it is to sort out the three beams.Stated generally in another way, the greater the convergence angle theless the possibility that a given beam will impinge upon a color-areaother than the one upon which it should impinge.

In agreement with the above mentioned rule, colorpurity was achieved inearly directional type color-kinescopes etc. by mounting the three guns120 apart, each in a separate neck, with separate beam-deflecting coilsand keystone-correction means associated with each gun. (As to this seethe Goldsmith patent, for example.)

Schroeder (U. S. Patent 2,595,548), by bringing the guns close together,delta (A) fashion, in a single neck,

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dispensed with separate beam-deflecting coils and eliminated thenecessity for keystone correction. In Schroeders tube, in order toreduce color-dilution occasioned by possible overlap of the three beamspots at the screen, the practice has been to make the three beams, andthe ap'ertures in the mask, of a diameter considerably smaller than thebeams and apertures which can be used in a similar tube havingwidely-spaced guns. Other gun parameters remaining the same, anyreduction in either of said diameters results in a decrease in thequantity of electrons available at the screen and a consequent decreasein the brightness of the color-pictures reproduced on said screen. 7

Accordingly, the principal object of the present invention is to providea method of and means for producing images of improved brightness in C.R. tubes of the kind wherein the beam electrons are subjected to a firstfocusing action near their source and to one or more independentfocusing actions near the screen or target of the tube.

Another and related object of the invention is to achieve the foregoingobject, in a multi-gun color television tube (a) without any sacrificein color-purity, (b) with out any increase in the spacingor convergenceangle of the electron-guns and (0) without adversely affecting thelimiting resolution.

The present invention teaches that when a dual-focus cathode-ray tubeemploys a screen unit of the line-screen type (as distinguished from thedot-screen type), it is not necessarily the diameter, but rather thewidth of the beam at or near its plane-of-deflection that determinescolor purity. By beam-width, in the case of a line'- screen tube, ismeant the dimension, in the plane of the screen that extendsapproximately at right angles: to the direction of extension ofcolor-phosphor lines. The significance of the difference betweenbeam-diameter and beam-width as a color-purity determinant will beapparent when it is appreciated that a 20% reduction in beam-diameterresults in' a 36% reduction in the bea'rns cross-sectional area and anapproximately equivalentre duction in beam-current and image-brightness,whereas a 20% reduction in beam width only provides the same increase incolor-purity yet reduces the beams cross-sec tional area only 10.5%.Consequently, a substantial reduction in loss of brightness results.

Thus, it will be seen that the achievement of the previ'-' ouslymentioned and other objects of the invention is predicated, first, uponan appreciation of the phenomena involved in the independent focusingactions which take place in the C. R. tubes to which the invention isappli= cable and, second, upon the realization of the fact' that optimumperformance cannot be achieved in such tubes with a beam (or beams) ofconventional circular crosssectional contour.

The particular shape of the beam (or beams) required to achieve optimumperformance in dual-focus tubes in-' volves a consideration not only ofthe particular type of electron-lens (e. g., cylindrical or sphericaemployed in the screen unit, but also of the pattern (e. g., dot orline) of the screen per s'e. In applying the invention to dual-focus,tri-color, 3-gun tubes of the line screen variety the invention dictatesthe use of three beams, each having a long cross-sectional dimensionand, preferably, two straight side's which, in theirplane-"ofdeflection, are parallel to the direction of extension of thecolor-lines on the screen.

Insofar as color-purity isconcerned the beams size and shape near itsplane-of-defiection are of considerable importance while its size andshape in sections successively closer to the focusing grill areo-fsuccessively less im- (For example, the size and the shape of the beamat the plane of the lens-grill have no effect Whatsoever on colorpurity; the only factor of importance with respect to color purity insaid plane is the direction of travel of the individual electrons in thebeam.) Accordingly, the beams may be endowed with their noncircularshape by an appropriately shaped and positioned stopping aperture which,for convenience, may be placed in each gun. If the electron-guns alsocontain a conventional lens-system the non-circular pattern of each beamin its plane-of-defiection is converted into a circular pattern by thetime the beam reaches the screen-unit. Thus, the present invention maybe said to teach a novel method of operating a C. R. tube which involvesscanning its screen-unit or target assembly with the circular end of abeam which, in its plane-of-deflection, is of noncircular contour.

The invention is described in greater detail in connection with theaccompanying two sheets of drawings, wherein:

Fig. 1 is a partly diagrammatic view in perspective of a tri-colorkinescope containing a color-screen unit of the so-called focusedline-screen variety and a battery of three electron-guns capable ofprojecting electron-beams of non-circular contour;

Fig. 2 is a longitudinal sectional view of the C. R. tube of Fig. =1, ona reduced scale;

Fig. 3 is an enlarged sectional view taken on the line 33 of Fig. 2showing the pattern of the three noncircular beams as they emerge fromthe battery of electron-guns;

Fig. 4 is an enlarged view of the three beams, taken on the line 44 ofFig. 2 in their plane-of-deflection or center-of-scan;

Fig. 5 is a similar view, taken on the line 55 of Fig. 2, showing thatthe non-circular beams of the earlier views assume a circular contour asthey approach the colorscreen unit;

Fig. 6, which is included to visualize the problem of color-puritytolerance" with which the invention is concerned, is a view inperspective, on an enlarged scale, looking through the line-screen of aC. R. tube similar to the one shown in Fig. 1 but employing aconventional (circular) beam;

Figs. 7 and 8 plot the paths of electrons, in dual focus tubes, fromemissive surfaces of ditferent areas, through the lens field of aparallel wire grill; these drawings being referred to in explaining theprinciple of the present invention;

Fig. 9 is a view in perspective, similar to Fig. 6, but showing how theoriginally non-circular beam of the present invention effects anincrease in color-purity tolerance at the screen;

Fig. 10 is a partly diagrammatic view in perspective of acolor-kinescope of the dot-screen shadow-mask" variety, embodying theinvention; and

Fig. 11 is an elevational view of a hexagonal beamforming apertureconstructed in accordance with the principle of the invention for use inthe shadow-mask tube of Fig. 10.

The color-kinescope shown in Fig. 1 comprises an evacuated envelope 1having a main chamber 3 in the form of a frustrum which terminates atits large end in a window 5 through which the obverse face of the glassviewing screen 7 of a bi-part target assembly or screenunit 7, 9 isviewed. The viewing screen 7, here illustrated, is of the mosaicline-screen variety described in the Flechsig French Patent 866,065) andSchroeder (U. S. Patent 2,595,548) patents. It is provided on its rearor target surface with a multiplicity (say 1500 or more) of parallelydisposed phosphor lines R (red), B (blue) and G (green) of differentcolor-emissive chanacteristics arranged in a repetitive pattern ingroups of three. These parallel lines R, B and G are here shown asextending in the vertical direction, they may, however, extendhorizontally across the screen, or at an angle with respect to saiddirections. An electron-transparent light-reflecting 4 t film 11constituted, for example, of evaporated aluminum renders the entiretarget surface of the screen conductive. The other element of the screenor target assembly comprises an electrically conductive mask or grill 9made up of fine wires (say 0.003" diameter) disposed parallel to thecolor-phosphor lines on the screen 7, there being one opening in thegrill 9 for each group (R, B and G) of color lines. Separate externalleads 13 and 15 from the conductive surface (11) of the screen and fromthe grill (9), respectively, permit the application of appropriatepotentials from a source 17, to these separate electrodes. In theinstant embodiment it will be assumed that the metal coating 11 on thescreen is maintained at a potential approximately four times that of thegrill 9 in order to establish an electron-lens field in the Spacesbetween adjacent wires in said grill. The lines of force of which saidfields are comprised are not here illustrated since it is well knownthat with the described voltage distribution the lens-field adjacent toeach pair of wires is that of a cylindrical-lens the generatrices ofwhich, in this embodiment, are substantially parallel to the color lines(R, B and G) on the line-screen or target 7. As a consequence, the lensaction adjacent to the grill wires is such as to converge beam-electronstoward the long central axis of each of said lines, as is described ingreater detail in connection with Figs. 7 and 8.

The other or small end of the main chamber 3 terminates in a tubularneck 19 which contains a battery of three electron-guns 21, 23, 25 eachone of which is allotted to a particular screen color. The guns as hereshown are arranged delta (A) fashion about an axis normal to the planeof the viewing screen 7. They may, however, be arranged in line-as inthe Flechsig patents. As in Schroeder U. S. Patent 2,595,548, therequired horizontal and vertical scanning movements are supplied to eachelectron-beam by a common deflecting yoke 27 which will be understood tocomprise two pairs of electro-magnetic coils (27a, 27b, Fig. 10)disposed at right angles to each other on the neck 19 of the tube.

The guns here shown are of duplicate construction, and With theexception of the stopping aperture (41, 43, later described), are of thetype claimed by Hannah C. Moodey in copending application, Serial No.295,225. Thus, each gun comprises an indirectly heated cathode 29, acontrol grid 31, a short cup-like screen grid electrode 33, a firstaccelerating electrode (or first anode") in the form of a hollow metaltube 35 and a second accelerating electrode (or second anode) consistingof. a large tubular portion 37 common to the three guns. A conductivecoating 39 on the inner surface of the main chamber 3 and neck 19 of theenvelope 1 comprises a third accelerating electrode (or third anode).

As previously indicated, the present invention teaches that in 3-gundual-focus line-screen C. R. tubes maximum picture-brightness isachieved (without sacrificing colorpurity etc.) when a section of eachbeam (1', b and g) taken in its plane-of-defiection (44, Fig. 2) hasstraight sides a, a (see Fig. 3) or at least a long erosssectionaldimension along the axis x-x (same figure) parallel to the direction ofextension or the color-lines (R, B and G) on the screen-plate and ashorter dimension along an axis yy at a right angle to said long axis.(The planeof-defiection in three-gun tubes may be defined as the planein which the axis of each deflected beam, when extended rearwardly,intersects the axis of origin of that beam.) The means shown in Fig. 1for endowing each beam with a non-circular contour in itsplane-of-deilcction comprises a diaphragm 41 disposed within the firstanode cylinder 35 and having an appropriately shaped beam formingaperture 43 therein. The straight sides a and a and the long dimensionalong the cross-sectional axis x-x of the beams (r, b and g) areestablished by the stopping aperture 43 in each gun and continueparallel to each other throughout the space between the guns and theirplane-of-defiection 44 (Fig. 2). As

remiss indicated in Fig. 4, the beams r, b and g, retain their abovedescribed shape in their plane-of-deflection, but at said plane arereduced in area by the focusing action of the lens-system in each gun.The beams gradually lose their straight sides and acquire asubstantially circular contour (see Fig. as they approach the grill 9 ofthe screen-unit. This is so because the bi-potential electronlens fieldin the space between the first and second anode 35-37 of each gunoperates to focus, in or near the plane of the grill 7, an image of thecircular emitting surface of the cathode 29 or of the circularcross-section of the first cross-over (virtual cathode) in that gun. Asin light-optics, this focusing action is, of course, independent of theshape of any aperture near the lens.

As previously mentioned, the size and shape of the beams cross-sectionin its plane-of-deflection are of greater importance in determiningcolor-purity than the size and shape of other cross-sections on thescreen side of said plane. Why this is so will the more readily beapparent upon inspection of Fig. 6 which visualizes the problem withwhich the invention is concerned, and Figs. 7, 8 and 9 which illustratethe phenomena involved in the past and the present solutions of, theproblem.

Referring now particularly to Fig. 6 which is a greatly enlarged view ofa red electron-beam r as it passes through the apertures of a grill,indicated by the wires W W2, w and is focused upon two of the red linesR and R of a tri-color screen. Here the problem is indicated by showingthe focused split-ends of the beam to be of width-dimension d slightlylarger than that of the red colordines R, R so that the sides of thebeam overlap the adjacent green (G) and blue (B) lines on the screenand, consequently, produce color-dilution. The prior art solution ofthis problem is to reduce the diameter of the beam. This is visualizedin Figs. 7 and 8 which trace the paths of two beams r and r of differentsize (here disregard the cross-hatching on the large beam r', Fig. 8)from the plane-of-defiection 4-4, and thence through the space betweenthe grill-wires w W2 to a red line R on the screen. (In Figs. 7 and 8,in order to simplify the drawing, the diameter of the beam at the grillcorresponds to the spacing of the grill wires. As indicated in Figs. 6and 9 it may actually span the spaces between three or more adjacentwires.) In Fig. 7 the dimensions of the beam in its plane-of-deflection4--4 and at the screen S are smaller than the beam Fig. 8- at thecorresponding planes (44 and S). As a consequence, the beam in Fig. 7 isconfined to the red-phosphor line R. If, as previously mentioned, it isassumed that a 20% reduction in beam diameter is required to achieve thedesired color-purity tolerance, the resulting reduction in beamcurrentand image-brightness is the order of 36%.

Referring still to Figs. 7 and 8 it will be observed that both drawingsare marked with lines e e 2 and e which mark the paths traversed byelectrons from various parts of the beam in their transit from theplane-ofdeflection 44 to the screen. It will also be observed that, inagreement with laws of electron-optics, the beam spot on the screen isan inverted image, of the beam in its plane-of-defiection. Now, havingin mind that the color-line R on the screen is perpendicular to theplane of the drawing, it will be seen that the electrons that extendbeyond the red line R (Fig. 8) onto the blue and green lines (not shown)can be traced back to the top and bottom sides of the beam in itsplane-of-defiection, as is indicated by the shading on the beam in itsplane-ofdefiection 44. It follows that if the electrons that make uponly said (shaded) sides of the beam are eliminated from the beam by thetime it reaches its plane-ofdefiection, they will likewise be missingfrom the beam spot (or spots) on the screen, irrespective of the shapeof the beam as it approaches the grill. Accordingly, in contrast to theprior art which dictates a uniform decrease in the diameter of the beam,the present invention by the use of the non-circular shape beam-formingaperture 43 (Fig. l) eliminates only those electrons that cause thetrouble. As a consequence, as shown in Fig. 9, the beam is confined toits own particular color-areas on the screen and color-dilution or othercross-talk is eliminated. In the instant case the number of electronsavailable at the screen, instead of being decreased 36% (as in the citedexample) is decreased by but 10.5%. Thus the present invention, asapplied to a line-screen tube, effects a net gain of approximately 25%in image brightness as compared with a similar tube wherein the beam orbeams are of conventionally circular contour in theirplane-of-deflection. This advantage holds for a beam having uniformcurrent distribution. A comparable advantage is achieved with beamswherein the current density is greatest at the center of the beam.

Where, as is preferable, the spacing between grill wires is less thanthe diameter of the beam at the grill (that is, less than the spot size,the focusing action of the lens-field in the grill-screen space uponeach beam is much the same as it is upon the conventional beam of Fig.6. That is to say, the grill wires w W2, W divide the beam into two (ormore) segments which are each subjected to the cylindrical lens actionof the electric field in the grid-screen space and this lens actionfocuses said beamsegments upon the phosphor lines of the particularcolor to which that beam is allotted. However, since the noncircularshape beam-focusing aperture in each gun has deprived the beam of theelectrons which have been shown (in Fig. 8) to produce color-dilutionthe beam is concentrated on said lines (as shown in Fig. 9) and no partof it impinges upon the adjacent lines of different colors.

The invention is not limited in its useful application to focused-beamline-screen tubes of either the single or plural grill varieties but maylikewise be employed to advantage in so-called shadow-mask tubes of boththe line-screen and dot-screen varieties.

In Figs. 10 and 11 the invention is shown as applied to shadow-maskdot-screen" tube. See Goldsmith 2,630,542 and Schroeder 2,595,548.) Herethe apertures 51 in the masking plate 53 and the circular phosphorcolor-dots R, B and G on the screen 55 are each arranged in a hexagonalmosaic pattern. That is to say, each mask-aperture and each color-dot issurrounded by six others; there being one group of red (R), blue (B) andgreen (G) phosphor dots for each mask-aperture. In this case, thebeam-forming aperture 57 in the first anode of each of the threeelectron-guns is stopped down from six sides (instead of two) to a shapeapproaching that of a hexagon (see Fig. 11) and the size of the circularholes in the mask may be increased a similar amount, without causingcolor-dilution, to permit the passage of a greater number of electrons.In this case, assuming a beam of uniform density the maximum possibleadvantages obtainable is only about 10% increase in brightness. Theapproximately 10% increase in brightness is due to the fact that thehexagons area is roughly 10% greater than that of circular beam thatwould have to be used to achieve the same freedom from color-dilution orcross-talk.

In applying the invention to plural-grill C. R. tubes of the kindwherein two parallel-wire grills are disposed at right-angles to eachother, the long-axis or" the noncircular beam-forming aperture should bedisposed parallel to the wires of the focusing grill. That is to say, asin Fig. 1, the long dimension of the non-circular beamforming aperture43 should extend parallel to the colorlines R, B and G on the screen.

From the foregoing description it should now be apparent that thepresent invention provides a novel method of and means for producingimages of enhanced brightness in dual-focus C. R. tubes, and this toowithout (a) any sacrifice in color-purity or other cross-talk, (b)without any increase in the spacing or convergence angle of theelectron-guns, where a plurality of guns are employed, and withoutadversely affecting the limit ing resolution.

What is claimed is:

1. Method of operating a CR. tube of the kind containing aplane-of-deflection and an apertured electrode through which beamelectrons successively pass in their transit from an electron-gun to theray-sensitive target surface of a mosaic screen, said method comprising,deriving an electron-beam of non-circular cross-section from saidelectron-gun, projecting said non-circular beam upon saidplane-of-deflection, and scanning said target-surface with said beam asmodified in its cross-sectional contour by the presence in its path ofsaid apertured electrode.

2. Method of operating a C. R. tube of the kind containing aplane-of-defiection through which beam-electrons pass in their transitfrom an electron-gun to the target surface of a screen electrode, saidmethod comprising; deriving an electron-beam of non-circular crosssection from said electron-gun, projecting said non-circular beam uponsaid plane-of-deflection, establishing a beam-focusing field adjacent tosaid target-surface, and scanning said target surface with said beam asmodified in its cross-sectional contour by the presence in its path ofsaid focusing field.

3. Method of operating a C. R. tube having a planeof-defiection throughwhich beam-electrons pass in their transit from an electron-gun to ascreen-unit of the kind comprising spaced-apart field and screenelectrodes, said method comprising; deriving an electron-beam ofnoncircular cross-section from said electron-gun, projecting saidnon-circular beam upon said plane-of-deflection, establishing abeam-focusing field in the space between said field and screenelectrodes, and scanning said screenelectrode with said beam as modifiedin its cross-sectional contour by the presence in its path of said fieldelectrode and said focusing field.

4. Method of operating a C. R. tube having a planeof-defiection throughwhich beamelectrons pass in their transit from an electrongun to ascreen-unit of the kind comprising a parallel-wire grill arranged withits wires parallel to the elemental ray-sensitive areas on the nearbytarget surface of a mosaic line-screen, said method comprising; derivingfrom said gun a non-circular elec tron-beam having a longcross-sectional dimension which is parallel to the line-like elementalareas of said screen, projecting said non-circular beam upon saidplane-ofdefiection, establishing a cylindrical lens-field in the spacebetween said grill and said screen, and scanning the linelike targetsurface of said mosaic screen with said beam as modified in itscross-sectional contour by the presence in its path of said grill-wiresand said cylindrical lensfield.

5. Method of operating a C. R. tube having a planeof-defiection throughwhich beam-electrons pass in their transit from an electron-gun to ascreen-unit of the kind comprising a punctate-aperture mask disposedwith its apertures in register with the circular dot-like ray-sensitiveareas on the target surface of a nearby mosaic dotscreen. said methodcomprising; deriving from said gun an electron-beam of hexagonalcross-section, projecting said hexagonal electron-beam upon saidplane-of-defiection, and scanning the dot-like target surface of saidmosaic screen with said beam as modified in its cross-sectional contourby the presence in its path of said punctateaperture mask.

6. A cathode-ray tube comprising a screen-electrode having a mosaictarget surface, an electrode mounted adjacent to said screen-electrodeand containing a pattern of apertures corresponding to the pattern ofelemental areas of said mosaic, an electron-gun mounted in a position toscan said apertured electrode and said screen-electrode, saidelectron-gun comprising a source of electrons, a diaphragm containing anon-circular beamforming aperture and an electron-lens system forconverting said non-circular beam into a beam of substantially circularcross-section prior to impinging upon said apertured electrode.

7. In a cathode-ray tube of the kind containing a planeof-defiectionthrough which beam-electrons pass in their transit from an electron-gunto the screen of a lenticular screen-unit, said electron-gun comprisinga source of electrons, a diaphragm having a non-circular beam-formingaperture operatively associated with said source and an electron-lenssystem effective in the region between said plane-of-deflection and saidscreen-unit for converting said non-circular beam into a beam ofcircular crosssectional contour.

8. The invention as set forth in claim 7 and wherein the screen of saidlenticular screen-unit is of the linescreen variety, and saidnon-circular beam-forming aperture has a long dimension extending in adirection substantially parallel to the direction of the lines on saidline screen.

9. The invention as set forth in claim 7 and wherein the screen of saidlenticular screen-unit is of the dotscreen variety, and saidnon-circular beam-forming aperture is of hexagonal contour.

10. In a 3-gun tri-color kinescope of the kind containing aplane-of-deflection through which electrons from the three guns pass intheir transit to a screenunit of the kind comprising a parallel-wiregrill disposed with its wires parallel to phosphor lines of threedifferent color-response characteristics on the nearby target-surface ofa mosaic line-screen, the improvement which comprises: a diaphragm, ineach of said guns, containing a non-circular beam-forming aperturehaving a long cross-sectional dimension substantially parallel to saidcolor-phosphor lines, electron-optical means operatively associated witheach diaphragm for projecting the non-circular beam resulting from thepassage of electrons through the aperture in that diaphragm upon saidplane-of-deflection and adapted to convert each non-circular beam into abeam of substantially circular cross-section during its transit throughthe space between said plane-of-deflection and said parallel-wire grill,means for establishing a cylindrical lens-field in the space betweensaid grill and the target surface of said screen, and means for scanningthe color-phosphor lines on said target surface with said beams asmodified in their cross-sectional contour by the presence in their pathsof said grill-wires and said cylindrical lensfield.

l.1. In a 3-gun tri-color kinescope of the kind containing aplane-of-defiection through which the three beams from said guns passalong different angularly re lated converging paths in their transit toa screen-unit of the kind comprising a target-surface made up of amultiplicity of groups of substantially circular phosphor covered dotsof difierent color-emissive characteristics and an electrode containinga multiplicity of substantially circular apertures disposed in a patterncorresponding to the pattern of the groups of color-phosphor dots onsaid target surface; the improvement which comprises: means operativelyassociated with each of said guns for imparting a hexagonalcross-sectional shape to each beam prior to its passage through saidplane-ofdefiection and for converting each of said hexagonally shapedbeams into a beam of substantially circular cross-sectional shape as itapproaches said screen-unit.

References Cited in the file of this patent UNITED STATES PATENTS2,595,548 Schroeder May 6, 1952 2,609,516 Flory Sept. 2, 1952 2,659,026Epstein Nov. 10, 1953

